In the past, fresh water was often readily available and cheap, because it was considered an unlimited resource and easily replenishable. Despite technological advances and better management, it has become evident that fresh water supplies are much harder to maintain than previously thought and that it is strongly dependent on spatial and temporal variation of precipitations. One of the most common techniques used to mitigate the reliance on long term weather predictions and precipitations is to construct big storage in natural or artificial tanks and lakes. But due to high temperatures and arid conditions in many southern climates, the evaporation losses have been found to be important. Therefore, it is very important to minimize evaporation losses in those storage water bodies. When you consider that less 0.1% of the freshwater on the planet is stored in lakes and man-made manufactured structure and the predictable scarcity of water, the need for prevention of large evaporation losses is of great significance.

Nowadays, it is estimated that 70% of worldwide water use is for irrigation, with 15-35% of irrigation withdrawals being unsustainable. It takes between 2,000 and 3,000 litres of water to produce enough food to satisfy one person’s daily dietary need, compared to only 2 to 5 liters required for drinking purposes. Another 22% is used in the industry. The main industrial users include hydroelectric dams, thermoelectric power plants, which use water for cooling, ore and oil refineries, which use water in chemical processes (cyanide ponds), and manufacturing plants, which use water as a solvent and cooling. The last 8% is used for household purposes. All water users share one common point: 95% of all the fresh water withdrawals are pulled from large water storing sites which suffer from excessive evaporation.

In the past years, intense agricultural practices, rapid increase in population, industrialization and urbanization is resulting in water scarcity. The situation is becoming more critical in arid and semi arid regions especially during droughts, when general scarcity of water is compounded by high evaporation losses from open water surfaces of lakes and reservoirs. In 2011, Texas experience the worst single-eyar drought in history, with entire reservoir going dry, large fish kills and complete loss of crops. During the severe droughts Europe (a typically temperate climate) witnessed in 2012, electrical power plants were shut down due to lack of cooling water and grain prices soared 20%, affecting the food supplies.

Assessment shows that by 2025, our water surplus will be only 2.7% of all utilizable water resources (meaning we will be using 97.3% of all that is available!). With today’s strain on water resources, it is imperative to conserve water by reducing evaporation losses, especially in drought prone area. Since most of our water comes from lakes and reservoir, controlling evaporation on those is where we should focus most of our efforts.

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Chemical WER (fatty alchohol) form a thin film have been found to be effective for reducing evaporation (some studies cites 46% reduction) and fairly safe to the environment (biodegrable and permeable to oxygen). However, wind velocities and temperature does affect them and they are generally not used when winds exceed 10-15 km/h or when the temperatures exceed 40-42C. They are also expensive to purchase and apply and they fall into a short term measure.

Reducing the exposed water surface works best if some shallow portions in the reservoirs can be easily separated from the rest of the lake. The method consists in building dykes to isolate the shallow portion of the reservoir and divert the water to deeper area and is somewhat effective in drought prone areas. Evaporation losses are eliminated in those section, thus reducing the evaporation losses over the entire reservoir. Unfortunately, it is dependent on the reservoir topography and can be somewhat costly to implement.

Wind breakers are another method that can only be employed on small water bodies. It consists in planting trees or other plants that acts as wind breakers, thus reducing the movement of air over the water surface, greatly reducing evaporation. On small ponds, trees can also provide some shade which will further reduce evaporation. However, this solution is limited to small reservoir. On big reservoirs, the wind breaker effect is limited to a short distance from the rim of the reservoir, leaving the inner water exposed.

Covering the surface of water bodies with a fixed or floating covers considerably reduces evaporation losses. These covers reflect energy inputs from the atmosphere and create a layer that prevent transfer of water vapour to the atmosphere. Fixed covers have been used for small and medium storage. They have the advantages to be somewhat affordable. One of the main drawback is durability as dust, leaves and other debris accumulate on top. They are also prone to tear (especially under heavy snow loads or heavy rains) and maintenance is expensive. For large water surfaces, floating mat and spheres have been used. It has been reported that small floating polystyrol sphere reduced evaporation by 80% in experimental tanks. Further studies using larger 100mm polyethylene sphere have also shown reduction in evaporation by 90%. Both spheres have the added advantage to reflect solar energy and eliminating potential algae bloom. To learn more about the potential harmful effects of algae, visit pond-algae-control.com.

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